1440p 120hz should give the lower input lag. I think you can´t use VRR with 120hz on Xbox, not sure about that. If you could, it would decrease the input further because on these Samsung VA PAnels, activating VRR decreases the input lag by some margin. However not a big margin enough to beat 120hz with a 60hz signal.

Also it doesn´t matter if your monitor shows 120hz on the OSD, it will only be at 120hz if Xbox says so.